Thermal and stress mapping of body lumens

ABSTRACT

The inventive method requires advancing a three-dimensional imaging balloon catheter to the site of a lesion to be imaged, inflating or molding the balloon to image the lesion, deflating the balloon, withdrawing the catheter from the body lumen and re-inflating the balloon which reassumes its memorized shape. Stress and thermal mapping of the balloon is then done by direct observation or by numerical analysis of the material strain and color of the re-inflated 3D imaging balloon.

[0001] This application is a Continuation application from applicationSer. No. 08/951,769, the contents of which is hereby incorporated byreference.

BACKGROUND OF THE INVENTION

[0002] The present invention relates to thermal and stress mapping of abody lumen, and more particularly to analyzing the material strain andcolor of the surface of a lesion molding balloon to classify the type oflesion and whether it is vulnerable to rupture.

[0003] It is widely recognized that plaques or lesions can be classifiedinto three broad categories: calcified or hard plaque lesions, fibrousor soft lesions and inflamed soft lipid filled plaques or lesions. Thediagnosis of the type of lesion drives the particular treatment of thelesion, whether it is removal of the lesion by rotablator, predilatationby balloon angioplasty, delivery of a stent, with or withoutpredilatation, or the like.

[0004] In particular, the identification of inflamed plaques or lesionsis important since these lesions are at greatest risk of rupture, whichcan lead to a large thrombus or blood clot, which can completely occludethe flow of blood through the artery, leading to injury of the heart orbrain. An inflamed or vulnerable lesion is characterized by its capthickness, lipid pool size and inflammation or temperature. This isdiscussed in great detail in WO 97/10748 published Mar. 27, 1997 andentitled “Detecting Thermal Discrepancies In Vessel Walls”, the entirecontents of which are hereby incorporated by reference. As discussed inthe published PCT application, considerable evidence indicates thatplaque rupture triggers 60-70% of fatal myocardial infarctions. As iswell known in the art, and described in the published PCT application,an inflamed plaque is hotter than the surrounding tissue. This publishedPCT patent application relates to using an infrared fiberoptic system tolocate inflamed heat producing plaque. However, the device described inthis PCT published application is very expensive, making it available ina limited number of procedures. What is needed is a more inexpensivemethod for classifying plaques or lesions, and in particular determiningwhich plaques are hard, soft or inflamed, which drives the treatmentafter diagnosis.

SUMMARY OF THE INVENTION

[0005] Applicants have discovered that in addition to providing athree-dimensional (3D) image of the geometry of a lesion or plaque in abody lumen, the balloon can be further analyzed to determine itsmaterial stress, which can in turn be used to determine the temperatureof a lesion as well as the hardness of a plaque or lesion. Differencesbetween the observed material stress and a baseline material stress canalso determine temperature differences along the balloon surface, whichcan be used to aid in determining whether a particular lesion isinflamed and vulnerable to rupture. The material stress of the balloonmaterial manifests itself in the color pattern observed under whitelight with a polariscope.

[0006] The inventive method requires advancing a three-dimensionalimaging balloon catheter to the site of a lesion to be imaged, inflatingor molding the balloon to image the lesion, deflating the balloon,withdrawing the catheter from the body lumen and re-inflating theballoon which reassumes its memorized shape. Stress mapping of theballoon is then done by analyzing the material strain and color of there-inflated 3D imaging balloon.

[0007] Direct observation of the color pattern which is predominantlyblue/green is considered indicative of an inflamed lesion. More precisetemperature mapping may be performed by digitizing the surface geometryof the balloon and computing the color pattern based on the digitizedsurface geometry and the observed narrowest balloon diameter ID_(i). Bycomparing the differences between a computed color pattern, which isbased on the digitized surface geometry of the balloon, to the observedcolor pattern, a temperature map can be generated which can be used todetermine whether a lesion is an inflamed vulnerable lesion which is atgreatest risk of rupture. The color pattern may also be used todetermine whether the lesion is a hard calcified lesion.

[0008] The comparison of color patterns may be done with a flip-chart ofbaseline images or by using a computer to match the actual color patternto a baseline image.

[0009] In addition to using a polariscope and analyzing the colorpatterns, an alternate embodiment of the inventive step would utilize atemperature sensitive balloon material which would change colordepending on the temperature of the body lumen, which could vary alongits axial length. Using such a temperature sensitive balloon materialwould allow direct observation of the withdrawn balloon which wouldprovide a temperature map of the body lumen including a lesion, whichwould then indicate directly whether the lesion was inflamed byobserving whether the temperature was higher than the normal bodytemperature.

BRIEF DESCRIPTION OF THE FIGURE

[0010] The file of this patent contains at least one drawing executed incolor. Copies of this patent with color drawings will be provided by thePatent and Trademark Office upon request and payment of the necessaryfee.

[0011]FIG. 1 shows a three-dimensional imaging balloon which has beenmolded to memorize the geometry of a portion of a body lumen containinga lesion or plaque;

[0012]FIG. 2 is a block diagram showing the components used in themethod, and

[0013]FIG. 3 is a color photo showing the color pattern produced by thepolariscope of calcified, soft and inflamed lesions or plaques.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0014]FIG. 1 shows a three-dimensional imaging balloon 10 which has beeninflated in a body lumen, memorized the geometry of the body lumenaround a lesion, been withdrawn from the body and reinflated to reassumeits memorized shape. Copending application Ser. No. 08/857,791 filed May16, 1997 describes the three-dimensional imaging balloon 10 (lesionmolding balloon) of the preferred embodiment, and its entire contentsare hereby incorporated by reference.

[0015] The balloon 10 is preferably constructed of semi-crystalline oramorphous blend materials, and more preferably of a blend of PBT andPETG, with a ratio of PBT to PETG ranging from 1:1 to 1:19, by weight.In the preferred embodiment the ratio is 1:3 or 25% PBT and 75% PETG, byweight. The preferred balloon material will deform or yield atapproximately 1 atmosphere. The preferred balloon is preferably used inconnection with lesions which produce an ID_(i) of between 2.5 mm and4.0 mm, as discussed further below.

[0016] In use the catheter carrying balloon 10 is tracked to the site ofa lesion, expanded to image the geometry of the body lumen, deflated andwithdrawn from the body. Typically the balloon 10 will be inflated for 1minute at 1.5 atm. Upon reinflating outside the body the balloon 10 willreassume its memorized shape, providing a three-dimensional image of thegeometry of the body lumen and lesion.

[0017] The reinflated balloon may be measured directly to determine thenarrowest diameter of the balloon ID_(i) and the maximum diameter of theballoon ID_(o), shown respectfully at 12 and 14 in FIG. 1. The preferredembodiment has been formulated to work best where the ID_(i) is in therange of 2.5 mm-4 mm.

[0018] The balloon material used to construct the three-dimensionalimaging balloon (3-D balloon) are anisotropic, and exhibit opticalbirefringence, which means that light entering the material travels atdifferent speeds along the material principal axes. In the case of theballoon 10, the two axes are the longitudinal and circumferential axes.The relative retardation of light along the two principal axes isproportional to the difference in internal stress loadings along the twoaxes, which is know as the stress-optic law. For a discussion of thetheory of birefringence please see Experimental Stress Analysis, 2ndEdition, James W. Dally and William F. Riley, McGraw-Hill, New York,1978.

[0019] The reinflated balloon is viewed through polariscope 16, as shownin FIG. 2. In a polariscope, light is first linearly polarized bypassing it through a filter, the filtered light then traverses theballoon material and is finally viewed through a polarizing filter whichis placed at right angles to the incident filter. Birefringent materialsrotate the plane of polarization, so that some portion of light exitsthe polariscope. If no material or non-birefringent material werepresent no light would pass through the polariscope. Monochromatic lightpassing through the polariscope appears as patterns of dark and lightcaused by the optical interference of the two principal axes lightwaves. Viewed in white light, the patterns appear as colored bands orregions on the balloon material (best seen in FIG. 3). The color patternchanges as the material object is rotated within the polariscope,therefore to enable proper comparison a fixed viewing angle ororientation must be used in viewing balloon 10 under the polariscope.Given a fixed viewing angle or orientation the color observed at a givenpoint on the balloon material will be determined by the relativeretardation, or the relative stress between the optical axes, thelongitudinal and circumferential axes of the balloon 10.

[0020] Balloon 10 is prestretched longitudinally which uniaxiallyorients the balloon material, inducing internal stress along thelongitudinal axis. When the balloon 10 is inflated or “molded” in thebody lumen the longitudinal stress is relieved and circumferential orhoop stress increases with increasing balloon diameter. Therefore, themolded balloon 10 shows a color which depends upon its diameter, andwhen the balloon is molded with a lesioned vessel of variable geometryand diameter, a pattern of colors appears in the polariscope. When theballoon is molded in a body lumen which has been heated by even a fewdegrees C (caused for example by an inflamed lesion), the elevatedtemperature reduces stresses built up in the balloon material, and thecolor pattern changes to shorter wavelengths indicating lessretardation. If the lesion is heated in localized spots, these spotswill show shorter wavelengths than the surrounding areas.

[0021]FIG. 3 shows three different balloons molded under differentconditions. Reference numeral 20 shows the image produced under apolariscope of a hard calcified lesion, 5 mm long, with ID_(i)=2.5 mm,ID_(o)=5 mm and the lesion temperature at 37° C. The balloon shown at 22shows the image produced under a polariscope of a soft fibrous lesion, 5mm long, with ID_(i)=2.75 mm, ID_(o)=5 mm and the lesion temperature at37° C. The balloon shown at 24 shows the image produced under apolariscope of the same soft material, but with a lesion temperature at40° C. to simulate an inflamed lipid filled vulnerable lesion, 5 mm longwith ID_(i)=2.75 mm, ID_(o)=5 mm.

[0022] Applicants have observed that inflamed lesions shift the colorpatterns to a blue/green pattern as shown at 24, which is quitedistinctive and different than the color patterns shown at 20 and 22. Itis believed that over a range of diameters, the blue/green color patternsuch as shown at 24 will only be caused by an inflamed lesion, which ishotter and shifts the wavelengths to the blue/green color patternobserved at 24.

[0023] Therefore, direct observation of the balloon material under thepolariscope can allow diagnosis of whether the lesion is inflamed bydirectly observing the color pattern and visually determining thetemperature map of the balloon surface.

[0024] An alternate embodiment of the inventive method would utilize atemperature sensitive balloon material, or coating or film covering theballoon material, which would directly change color depending on thetemperature of the body lumen coming into contact with the balloonmaterial. This would allow a direct observation of the temperature mapof the body lumen and lesion by simply viewing the balloon materialafter withdrawal from the body, eliminating the need for a polariscopeor the more elaborate computation steps discussed below.

[0025] As can be seen the balloon at 20 has a predominately solid purplecolor indicating a lesion whose diameter is axially the same along thelesion length. Testing has indicated that there may be some correlationbetween color and lesion hardness, with the shorter wavelengths ofpurple indicating a hard lesion and the longer wavelengths of yellow andorange indicating a softer lesion. This may be due to the greatermaterial stress caused by expanding the balloon against a hard lesionversus a soft lesion, which affects the relative stress between theoptical axes of the balloon material.

[0026] In a preferred embodiment, the balloon material is viewed underthe polariscope and a determination made as to whether the lesion isinflamed by observing the blue/green color pattern. It is believed thatthe temperature affects to the color pattern outweigh the diametereffects to the color pattern, since blue/green has only been observed inconnection with inflamed lesions, even across a larger range ofdiameters.

[0027] To more precisely determine a temperature map of the lesion, thegeometry of the surface of the reinflated three-dimensional imagingballoon 10 is digitized and input into computer 32. A baseline orcomputed color pattern may be calculated based solely on the geometry ofthe balloon material, given the color observed at the narrowest diameterID_(i) and assuming that strain is proportional to stress in the balloonmaterial. This computed color pattern only takes into account thediameter changes which affect the color pattern. Any differences betweenthe actual observed color pattern and the computed color pattern arebelieved due to either hardness of the lesion or temperature of thelesion. Furthermore, it is believed that the temperature effectsoutweigh the hardness effects. Therefore, the differences in the colorpattern can be used to quantify the surface temperature distribution ofthe balloon material. A temperature map of the lesion could then begenerated and used to determine whether the lesion was hotter than thesurrounding body lumen, indicative of an inflamed lesion.

[0028] In another embodiment, a series of baseline images or photos 30can be prepared relative to the parameters which affect color pattern.For example a set of photos could be produced at 10 different diametersID_(i), hard versus soft, and at 4 different temperatures. Once theID_(i) is determined by measuring the reinflated balloon at itsnarrowest diameter, the user could visually compare the color patternobserved under the polariscope with a flip-chart of images at aparticular ID_(i). A match would identify the hardness and temperatureof the lesion. It has been observed that calcified lesions do notexhibit the higher temperatures associated with inflamed lesions. Theimages or photos could also be loaded into computer 32 and a patternmatching program could automate the matching of the observed colorpattern to a baseline color pattern.

[0029] The above Examples and disclosure are intended to be illustrativeand not exhaustive. These examples and description will suggest manyvariations and alternatives to one of ordinary skill in this art. Allthese alternatives and variations are intended to be included within thescope of the attached claims. Those familiar with the art may recognizeother equivalents to the specific embodiments described herein whichequivalents are also intended to be encompassed by the claims attachedhereto.

What is claimed is as follows:
 1. A balloon catheter for use inthermally mapping a portion of a body lumen, comprising: an elongatecatheter having a distal end and having a balloon near the distal end,the balloon having an axial length and including a temperature sensitivematerial which when expanded into contact with a portion of the bodylumen, will change color along its axial length based on the temperatureof the body lumen contacting the balloon material, whereby analyzing thecolor changes along the axial length of the balloon will permit thedetermination of whether a portion of the lumen is at a temperaturehigher than the normal body temperature.
 2. The balloon catheter ofclaim 1 wherein the temperature sensitive material is the balloonmaterial.
 3. The balloon catheter of claim 1 wherein the temperaturesensitive material is a coating applied to the balloon.
 4. The ballooncatheter of claim 1 wherein the temperature sensitive material is a filmapplied to the balloon.